Combustion devices



April 7, 1970 D. WIGG COMBUSTION DEVICES Filed Aug. 5, 1968 IO OOOOOnOoo Inventor LESLIE D. Ulfi a /ZJ After-days United States Patent 3,504,491 COMBUSTION DEVICES Leslie Desmond Wigg, Fleet, England, assignor to Minister of Technology in Her Britannic Majestys Government of the United Kingdom of Great Britain and Northern Ireland, London, England Filed Aug. 5, 1968, Ser. No. 750,085 Claims priority, application Great Britain, Aug. 10, 1967, 36,855/67 Int. Cl. F02k 3/10 US. Cl. 6039.72 9 Claims ABSTRACT OF THE DISCLOSURE A flame stabiliser intended for use in a reheat system comprises an open gutter formed by two concentric annular baflles set at an angle to each other to form a divergent annular passage, and an annular vane of aerofoil section set centrally relative to the passage and upstream of it. Air can be discharged from circumferential rows of holes in the surface of the vane so that a gas stream will be diverted around the gutter by a sheet-like stream of air. Eddies forming at the rear edges of the baffles set up a recirculation zone behind the stabiliser. In a variation, a vane is combined with one of the annular baflies.

The present invention relates to combustion devices wherein a flame stabilisation zone is formed in a swiftly moving gas stream by injecting a sheet-like screen of gas transversely to the direction of gas flow.

Such devices are generally known as aerodynamic flame stabilisers and are particularly suitable for use where combustion is required only occasionally.

In aircraft gas turbine jet propulsion engines, extra power can be developed by burning additional fuel in an exhaust reheater, or afterburner. It has long been the practice in such installations to use fixed baffles to produce stable turbulent (or recirculation) zones in their wakes wherein flames will maintain themselves. Such baflles offer undesirable resistance to flow through an after-burner duct towards a propulsion nozzle during such times as reheat is not in operation. One of the more common forms of fixed baflie is a V-shaped gutter having its apex pointing upstream of the flow, the extreme simplicity of the arrangement tending to outweight adverse considerations. By setting the arms to an included angle of the order of 45, or slightly less according to flow velocity, a reasonable optimum between resistance and the provision of an adequate recirculation zone can be obtained.

It has been proposed on numerous occasions to employ areodynamic flame stabilizers in afterburners for the creation of recirculation zones as required by injecting streams of air across flow through a duct but this has not found much favour for various reasons including the quantities of air required and its dilution effect on the gas stream.

The present invention seeks to combine areodynamic flame stabilisers with fixed baffle arrangements to provide adequate reheat performance coupled with low pressure loss through the system at other times.

A combustion device according to the invention comprises baflfles arranged in a fluid duct and inclined relative to flow through the duct so as to define a divergent channel and means disposed upstream of the divergent channel for discharging a stream of gas generally transversely of the duct to deflect flow through the duct away from the said channel and around the baflies.

In a preferred form of the invention, the means for discharging the gas stream comprises a hollow vane of aerofoil shape exposed to flow through the duct and having apertures in at least one surface through which gas may ice be discharged to induce the flow to separate from the surface.

Alternative embodiments of the invention will now be described by way of example with reference to the accompanying diagrammatic drawings, of which:

FIGURE 1 is an axial section through part of an afterburner duct including a flame stabiliser, and

FIGURE 2 is an axial section through part of another flame stabiliser.

The afterburner duct of FIGURE 1 comprises a cylindrical casing 1 extending between a turbine outlet and a jet propulsion nozzle (neither of which is shown), the direction of gas flow being from left to right as indicated by the arrows. A baffle ring 2 mounted coaxially within the duct comprises two concentric annular baflies 3, 4 of frustoconical section which have their surfaces inclined in 0pposite senses. The surface of the outer baflle 3 is inclined outwardly in the downstream direction while that of the inner baflle 4 is inclined inwardly. The baffles thus define an annular divergent channel within the duct.

Situated immediately upstream of the baffle ring, and coaxial with it, is an annular vane 5 (shown in two parts for convenience to illustrate alternative arrangements). The vane comprises a hollow ring of symmetrical aerofoil section, or substantially so, to which air may be supplied under pressure and may be of composite or one-piece construction as shown at the top and bottom of the figure respectively. In the first case, the vane is made up from a circular tube 5a with a rear fairing 5b. The diameter of the vane 5 is such that its cross-sectional axis will be midway between the baffles 3, 4 as indicated by the distance L on the figure.

Circumferential rows of closely-spaced holes 6, 7 and 8, 9 are drilled through the radially inner and outer surfaces of the vane and may extend radially through the said surfaces relative to the duct (as 6, 7) or be inclined outwardly of the vane in an upstream direction (as 8, 9).

In normal circumstances, gases passing along the duct will flow round the vane 5 and past the baffles 3, 4 with little disturbance as shown in the lower part of the figure. When reheat is required, high pressure air is supplied to the interior of the vane to be discharged from the holes in its surface. The gas flow along the duct is thereby induced to separate from its normal path over the vane and will be diverted towards the baflies 3, 4 by a curtain of air, as shown in the upper part of the figure. The effect of this will be equivalent to that of a solid gutter and recirculation will occur in the resultant wake. Fuel injected into the duct upstream of the stabiliser and ignited conventionally can thus be induced to burn and the flame will maintain itself during such time as air continues to be discharged from the holes in the vane.

The amount of air required with the arrangement just described will be considerably less than that which would be necessary to maintain a flame stabilising zone of comparable extent by purely aerodynamic means and may be readily tapped from the compressor of an associated gas turbine engine or supplied from an external source without excessive extra power requirements. The elfect of this air on combustible gas mixture in the afterburner will also be less pronounced.

The included angle between the surfaces of the bafiles is of the order of 30 in this embodiment. Encouraging experimental results have been obtained with such an angle and it is believed that an angle of 20 or so will be practicable for purposes of flame stabilisation with obvious advantages as regards reduced obstruction to flow at other times.

In the arrangement of FIGURE 2, a vane is combined with a baffle ring. An outer bafile 3 is used exactly as in the previous embodiment and an inner baflle 10, inclined as before is extended forwardly to form the radially inner surface of a hollow annular vane 11 of aerofoil section whose other surface passes through the channel formed between the baffles to terminate coincidentally with the downstream edge of the bafile 10. One circumferential row of holes 12 is provided in the radially outer surface of the vane 11 by which air can be discharged to divert gases flowing over that surface towards the bafi-le 3. Normally, there will be little disturbance to the flow over the vane but a recirculation will be induced as before when flow is diverted' Various detail modifications can be made to the constructions described without affecting their basic mode of operation. For instance, air might be discharged from the vanes through continuous slots rather than from rows of closely-spaced holes. Again, individual bafiles could be constructed of curved rather than fiat plate sections as shown, or they may be of a streamlined cross section.

I claim:

1. A combustion device comprising bafiles arranged in a fiuid duct and inclined relative to flow through the duct so as to define a divergent channel, and means disposed upstream of the divergent channel connected to discharge a stream of gas generally transversely of the duct to deflect fiow through the duct away from the said channel and around the bafiies.

2. A combustion device according to claim 1 comprising a hollow vane of aerofoil section exposed to flow through the duct and at least one aperture formed in the surface of the vane through which gas may be discharged to induce the fiow to separate from the said surface.

3. A combustion device according to claim 2 in which a bafile constitutes part of the hollow vane.

4. A combustion device according to claim 1 comprising two concentric annular baffies having surfaces inclined in opposite senses.

5. A combustion device according to claim 4 further comprising an annular hollow vane of aerofoil section exposed to flow through the duct, the cross-sectional axis of the vane extending along the axis of the divergent channel, and at least one aperture formed in the surface of the vane through which gas may be discharged to induce the flow to separate from the said surface.

6. A combustion device according to claim 5 having apertures extending through the surface of the vane substantialiy normal to the axis of the duct.

7. A combustion device according to claim 6 having apertures extending through the surface of the vane divergently inclined outwardly of the vane in an upstream direction.

8. A combustion device according to claim 1 comprising two concentric annular baffles having surfaces inclined in opposite senses, a hollow annular vane of aerofoil section disposed coaxially upstream of the bafiles with apertures formed in the radially inner and outer surfaces of the vane whereby gas may be discharged to induce flow through the duct to separate from the surfaces of the vane and pass around the baffles.

9. A combustion device according to claim 3 comprising two concentric annular baffies having surfaces inclined in opposite senses, wherein one bafile is extended in an upstream direction to define part of a hollow annular vane having a surface extending through the divergent channel and apertures formed in said surface of the vane whereby gas may be discharged to deflect flow through the duct around the second bafile.

References Cited UNITED STATES PATENTS 2,622,396 12/1952 Clarke 39.72 2,657,532 11/ 1953 Reid 6039.72 2,979,899 4/ 1961 Salmon 60-39.72 3,455,108 7/1969 Clare 60-39.72

FOREIGN PATENTS 964,510 7/ 1964 Great Britain.

MARK NEWMAN, Primary Examiner D. HART, Assistant Examiner 

